1. Field of the Invention
This invention relates to a method for the low temperature lamination of inert fluoropolymers to the surfaces of metals and printed circuit board (PCB) substrate.
When thermal grafting or graft copolymerization is carried out on the plasma pre-activated surface and interface of a fluoropolymer in contact with a metal surface in the presence of an adhesive, lamination occurs simultaneously. The graft polymerized layer can concurrently interact with the adhesive layer by crosslinking or other chemical or physical interactions, resulting in strong interfacial adhesion strength. The surface graft copolymerization with lamination of the metal is carried out under atmosphere conditions, and in the complete absence of any added polymerization initiator or system degassing.
Functional monomers used for thermal grafting or graft copolymerization with concurrent lamination include, but not limited to, epoxide containing monomers, hydroxy-containing monomers, amine-containing monomers, monomers of polyelectrolyte and monomers of polyampholyte. The adhesives used for lamination are mainly from the epoxy resin families.
The lap shear adhesion strength can readily exceed the tensile yield strength of the substrate polymer film. The lap T-peel adhesion strengths of the so laminated fluoropolymer-metal interfaces or fluoropolymer-PCB substrate interfaces are in excess of 8 N/cm, with delamination occurring in the bulk of substrate polymer film via cohesive failure. The strong adhesion between the fluoropolymer and the metal or PCB substrate is due to, on the one hand, the formation of covalent bonding of the grafted functional chains on the polymer surface, and on the other hand, the crosslinking or other strong interactions of these grafted chains to epoxy resin adhesive, which has strong adhesion towards metals and PCB substrate surfaces. The present invention distinguishes itself from the prior art in that the grafting/lamination process is carried out at temperatures substantially below the melting processing or sintering temperatures of fluoropolymer and that the adhesion strengths are higher than those in which only adhesive is used.
2. Description of Related Art
Fluoropolymers in general, and polytetrafluoroethylene (PTFE) in particular, are excellent candidates for dielectric materials in the application of packaging of microelectronics. They are chemically inert and they also have good thermal stability. With improved adhesion between a fluoropolymer and a metal or PCB substrate, the application of fluoropolymers in multi-chip module(MCM) packaging, for example, may become a reality. However, the physical and chemical inertness associated with most of the fluoropolymer dictates the use of more drastic means for achieving the required surface modifications. The strategies of surface chemical and physical modification have been widely implemented for fluoropolymers and hydrocarbon polymers alike.
One of the major drawbacks of the most commonly utilized technique of plasma treatment is that the physicochemical characteristics of the modified polymer surfaces, including surface compositions, are time-dependent. Chain and polar group reorientation in the surface region can result in gradual deterioration of the surface reactivity. Furthermore, anomalous changes in oxygen and fluorine contents, and therefore also surface compositions, may result from the presence of surface hydrocarbon contamination during plasma treatment, as suggested by M. A. Golub, E. S. Lopata, L. S. Finney, Langmuir, 10, 3629 (1994). To overcome the time-dependent surface characteristics, the plasma-treated fluoropolymers have been subjected to further surface modification via graft copolymerization, as shown in K. L. Tan, L. L. Woon, H. K. Wong, E. T. Kang and K. G. Neoh, Macromolecules, 26, 2832 (1993); E. T. Kang, K. G. Neoh, W. Chen, K. L. Tan, C. C. Huang and D. J. Liaw, J. Adhesion Sci. Technol., 10, 725 (1996); and Tie Wang, E. T. Kang, K, G, Neoh, K. L. Tan, C. Q. Cui and T. B. Lim, J. Adhesion Sci. Tech., 11, 679 (1997).
The patent literature contains numerous disclosures of surface modification of fluoropolymers for adhesion enhancement. However, most of the cases are related to plasma or chemical surface treatment. A few cases are related to surface modification via graft copolymerization. Almost no case study is directly related to the modification of fluoropolymer via surface graft copolymerizaiton for the improvement of adhesion between two fluoropolymer surfaces, between a fluoropolymer surface and a conjugated polymer surface, or between a fluoropolymer surface and a metal surface. Throughout our exhaustive patent literature search, there is no relevant process which involves the simultaneous modification of a fluoropolymer surface via grafting or graft copolymerization and the concurrent lamination of a metal in the presence of an adhesive. Almost no case study is directly related to surface graft copolymerization for adhesive enhancement.
For further background reading, reference is made to the following patent publications relating to surface modifications via grafting, and surface modifications for adhesion improvement:
Japanese Patent Publication Nos. 60,026,071A; 6,206,946A; 7,025,952A; 7,149,960; 7,186,325A, 60,020,941A; 92,004,353B; 1,265,886A, 7,186,325A; U.S. Pat. No. 4,506,035; Canadian Patent No. 2,136,897 and European Patent No. 656376.
For patent literature dealing with fluoropolymer-metal laminates, reference is made to the following patent publications:
U.S. Pat. Nos. 4,640,866; 4,861,408; 4,911,771; 4,996,097; 4,933,060 and 7,036,039.
European Patent Publication Nos. 160,418A; 160,439A; 160,439B; 331,429A and 463,105A.
Japanese Patent Publication Nos. 2,178,028; 5,259,635A2 and 63,170,992.
It is an object of the present invention to provide a new method for the direct lamination of fluoropolymers to metal and PCB substrate surfaces, as well as metal to PCB substrate surfaces, under atmospheric condition for microelectronic applications.
It is also an object of the present invention to improve the adhesion and affect the lamination in a simple process.
These and other objects and advantages of the present invention are obtained by providing a method for the modification of fluoropolymer via, first plasma pretreatment, followed by thermal graft copolymerization of an appropriate functional monomer and curing together with an adhesive or adhesives at the lapped interface between the fluoroppolymer and the selected metals or PCB substrate.
A desirable low grafting/lamination temperature is selected to be substantially below the melting or sintering temperatures of the fluoropolymer. Desirable radio frequency argon plasma with low plasma power is selected for the pretreatment of the fluoropolymer to minimize the undesirable overoxidation, etching or sputtering of the fluoropolymer surface.
The objects and advantages of the present invention can be achieved when the monomer used for surface graft copolymerization with concurrent lamination are selected from a group of vinyl monomers which contain epoxide or amine functionalities in the pendant group or groups, which can cure with the applied adhesive or adhesives to form a crosslinking network.
The objects and advantages of the present invention can be achieved when a pure monomer is used for graft copolymerization.
The objects and advantages of the present invention can be achieved on virtually all fluoropolymer substrates. The objects and advantage are realized on fluoropolymer substrates, such as, but not limited to, poly(tetrafluoroethylene) (PTFE) and its derivatives, composites and copolymers, including the particulate or fiber reinforced fluoropolymer composites, copolymer of tetrafluoroethylene and hexafluoro(propyl vinyl ether), copolymers of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole, and copolymers of tetrafluoroethylene and vinyl fouoride, poly(vinyl fluoride), poly(vinylidene fluoride), polychlorotrifluoroethylene, vinyl floride/vinylidene fluoride copolymer, and vinylidene fluoride/hexafluoroethylene copolymer.
The objects and advantages of the present invention are obtained when the selected metals for lamination are aluminum, copper, gold, platinum, and their alloys.
It is widely accepted that formation of covalent bonds between the polymer surface and the resin matrix can enhance the interfacial adhesion. The present invention is based on the fact that functional groups of the covalently tethered polymer chains on the fluoropolymer surface interact through chemical reactions, ionic interactions, or other chemical and physical interactions with the adhesive resins which have strong adhesion towards contacting metal surface to result in strong adhesion between the fluoropolymer and metal. The lamination temperature in the present invention is thus governed by the optimum temperature for surface graft copolymerization and curing of adhesive. This temperature is substantially below the melting point or the sintering temperature of the fluoropolymer.
In the preferred method, the argon plasma pretreated fluoropolymer surfaces are subjected to thermally induced graft copolymerization with reactive vinyl monomers containing the epoxide, amine, cationic, anionic or amphoteric functional groups. The plasma pretreatment are carefully controlled to introduce peroxide and hydroxyl peroxide species on the fluoropolymer surfaces to initiate the subsequent surface graft copolymerization, resulting in covalently tethered functional polymer chains on the fluoropolymer surface. When the surface graft copolymerization is carried out and the functional groups of the grafted chains are capable of forming strong interaction with adhesives of metal, strong adhesion of the metal to the fluoropolymer surface is achieved. Furthermore, the simultaneous grafting and lamination process at the polymer-metal interface is affected by thermal decomposition of the peroxides and hydroxyl peroxides at the fluoropolymer surface. The process can be carried out near the peroxide decomposition temperature and adhesive curing temperature which are usually less than 150xc2x0 C. and under atmospheric conditions in the complete absence of an added polymerization initiator. The joint delaminates by cohesive failure inside the fluoropolymer.
The preferred application and the best advantages of the present invention are obtained from fluoropolymer films, thin sheets or plates, as well as from metal foils, films, thin sheets or plates. Thus, in the preferred method, the surface of the fluoropolymer is first pretreated with radio frequency gas plasma. The selections of plasma type, plasma power and duration for pretreatment are important. High plasma power and long pretreatment time can result in excessive etching of the polymer surface, in surface crosslinking, and in dehalogenation of the polymer sample. The preferred range of plasma power range of treatment time is typically from 10 W to 50 W. The preferred range of treatment time is typically from 5 s to 120 s, and the preferred frequency is typically in the range of 5 kHz to 50 kHz.
Monomers for the simultaneous graft copolymerization and lamination process are selected from a group of vinyl monomers which readily undergo free-radical initiated polymerization. Desirably, the monomers are selected from the family containing the epoxide, amine, anionic, cationic or amphoteric functional groups.